Nonlinear semiconductor switches such as diodes are employed in various electronic control applications where it is necessary to switch current flow in response to a prescribed level of voltage. One such application involves visual display devices which employ a matrix of electrically energizable pixels (picture elements). The pixels may consist, for example, of well known liquid crystal elements in which a film or cell of liquid crystal material is positioned between a pair of electrodes. Conventional nematic type liquid crystals, which are commonly used in displays, consist of elongate molecules whose longitudinal axes can be rotated when an electric field or prescribed magnitude is applied to the material. The molecular rotation rotates the polarization of light transmitted through the liquid crystal material.
One common type of liquid crystal display employs twisted nematic crystals sandwiched between a pair of parallel glass sheets and a pair of polarizers respectively on opposite sides of the glass sheets. The polarizer on the input side of the display allows ambient light polarized in only one direction to pass into the display. The glass sheets have parallel lines formed by etching or the like on the opposing faces thereof. The molecules of the nematic crystal material near the surface of each glass plate tend to align their long axes parallel to the lines on the glass plates. The plates are oriented relative to each other such that the two sets of lines are nonparallel, e.g. 90 degrees off axis, thus giving the nematic crystals a helical or twisted orientation. Polarized light passing through the cell is rotated by the crystals. The second polarizer can thus allow the passage of the rotated light out of the cell. Application of an electric field across the cell rotates or "untwists" the molecules so that their axes extend substantially parallel to each other, thus blocking passage of light through the cell and the polarizers.
In order to display changeable data using liquid crystal displays, the pixels are arranged into M rows and N columns defining a matrix array that is addressed using conventional "X-Y" addressing techniques which employ M+N address lines. Each pixel possesses a unique X-Y location in the matrix which may be addressed by a corresponding combination of X and Y addressing lines.
Select voltages are subsequently applied to all rows of the matrix while the data voltages are applied to the columns. After scanning all lines in the display the polarity of the pixel voltages is reversed for the next scan. This prevents a DC component of voltage from being applied to the liquid crystal material which leads to degradation of the material. The liquid crystal pixel responds to the RMS voltage applied across its electrodes and the material, e.g. a threshold between V.sub.RMS =1.1 volts and V.sub.RMS =1.9 volts.
Direct multiplexing of a matrix type liquid crystal display is possible for only a limited number of rows (on the order of 10 rows) due to this poorly defined threshold, e.g. 90% transmissivity at V.sub.RMS =1.1 V; 10% transmissivity at V.sub.RMS =1.9 V. To increase the number of possible rows in a display, without losing overall display quality, a threshold device must be incorporated into each liquid crystal picture element.
The use of conventional diodes as isolating devices for pixel addressing has been less than completely satisfactory for several reasons. First, it is not always possible to control or select the threshold voltage at which the diode commences to conduct current when forward biased. Additionally, variations may exist in the threshold voltage of the diodes used in a single matrix. For a given level of address signal voltage, some of the forward biased diodes may conduct while others having a higher threshold voltage may not conduct or may conduct at a different rate. Accordingly, it is necessary to assure that a relatively high level address signal is applied to address the pixels.
It is also desirable to minimize the rise time of the current applied to each pixel in order to quickly switch the pixel after it has been addressed. The current-voltage characteristics of diodes previously employed for pixel isolation are such that the rise time of the current conducted by the diode following the application of threshold voltage is slower than desired.
Thin film field-effect transistors have also been used as isolation devices in liquid crystal pixel elements. Transistors have a more complex structure than do diodes however and are three terminal devices which complicate pixel manufacture.